Aerosolized particles created from vehicle exhaust and other contaminants can accumulate in the atmosphere and reduce the speed of winds closer to the Earth's surface, which results in less wind power available for wind-turbine electricity and also in reduced precipitation, according to a study by researchers from Stanford University and the National Aeronautics and Space Administration (NASA).

"These aerosol particles are having an effect worldwide on the wind speeds over land; there's a slowing down of the wind, feeding back to the rainfall too," said civil and environmental engineering Associate Professor Mark Z. Jacobson, co-author of the study with the late Yoram J. Kaufman from NASA Goddard Space Flight Center, who died in May 2006. "We're finding a reduction of rain, and that can lead to droughts and reduction of water supply."

Jacobson and Kaufman's study, based on NASA satellite data of aerosol accumulation, measurements of wind speeds over the South Coast Basin in California and in China, and computer model simulations over California as a whole and the South Coast Basin, was announced on Jan. 19. The researchers used both the model and data to study the effects of aerosol particles on wind speed and rainfall.

Aerosol particles floating in the atmosphere absorb or scatter solar radiation, and prevent it from getting to the ground. This cools the Earth's surface and reduces daytime vertical convection that mixes the slower winds found near the ground with the faster winds at higher altitudes. The overall effect is a reduction in the speed of near-surface winds, which Jacobson has calculated to be up to 8 percent slower in California.

Wind power made up 1.5 percent of the state's energy portfolio in 2005, according to the California Energy Commission. However, slower gusts may reduce wind's economic competitiveness compared to other energy sources, such as fossil fuels, according to the study's findings.

Aerosol particles may be responsible for the slowing down of winds worldwide. Wind supplies about 1 percent of global electric power, according to Jacobson. Slow winds may hinder development of wind power in China, where it's a needed alternative to dirty coal-fired plants. Aerosols' reduction of the wind also may explain the reduction in the Asian seasonal monsoon and "disappearing winds" in China, observations found in other studies. Moreover, slack air currents may hurt energy efficiency in Europe, where countries like Denmark and Germany have made major wind-power investments.

Slower winds evaporate less water from oceans, rivers and lakes. Furthermore, the cooling of the ground provoked by the aerosol particles reduces the evaporation of soil water, according to the study.

Additionally, the accumulation of aerosol particles in the atmosphere makes clouds last longer without releasing rain. Atmospheric water forms deposits on naturally occurring particles, like dust, to form clouds. However, if there is pollution in the atmosphere, the water has to deposit on more particles. Spread thin, the water forms smaller droplets. Smaller droplets in turn take longer to coalesce and form raindrops. In fact, rain may not ever happen, because if the clouds last longer they can end up moving to drier air zones and evaporating, according to the findings.